This invention concerns orthopaedic joint components, tools and methods more particularly for use in relation to the femur at the knee joint. The invention has in fact been conceived initially with a view to application as or for a femoral component of so-called unicompartmental form in a prosthesis according to U.S. Pat. No. 4,085,466. It is accordingly appropriate to describe the invention in more detail in terms of such application, but it is to be understood that the invention is more widely applicable to other forms of prosthesis.
In the present context "unicompartmental" qualifies a component discretely applicable to a femoral condyle to provide a replacement articular surface. While two such components can be used on the associated condyles of a joint, this distinguishes from a bicompartmental component in which a single one-piece femoral component has an associated pair of condylar parts connected by a bridging portion, and also from a tricompartmental component in which the bridging portion defines a patellar articular surface.
In the same context "surface replacement", used hereinafter, involves the provision of a component having a body largely of shell form and defining two principal surfaces in back-to-back disposition, one such surface being intended as the replacement articular surface and the other such major surface to interface with the relevant bone. In this case the bone is of course the femur at the knee and the respective principal surfaces are commonly grossly convex and concave.
Unicompartmental components presently in routine use include two principal types. In each type the articular surface is normally of polyradial shape, that i s to say the surface is deliberately non-spherical. This shaping appears to reflect confused thinking which assumes a need for a varying center of curvature for the articular surface in direct relationship with the fact that there is in the natural situation, a varying center of rotation between the femur and tibia during flexion-extension movement. Generation of the latter variation is of course appropriate following surgery if the natural joint function, including the action of retained ligaments, is itself to be closely simulated but study in development of the present invention finds no necessity for a related polyradial curvature.
The routinely used types differ, however, in the form of interface surface configuration used in relation to securement of the component to the bone. In one type the interface surface consists of a plurality of plane facets intended for securement, with or without bone cement, against a closely matched surface cut on the condyle. The other type is of surface replacement form and the corresponding interface surface closely follows the polyradial shape of the articular surface. This type is intended for securement with cement, the condyle being at best relatively coarsely prepared in terms of matching and the cement being deployed to fill gaps resulting from mismatching.
There are difficulties and disadvantages with both types. For the one type the related bone preparation is complex by virtue of the need to cut plural facets in a predetermined relationship to match the component. Moreover this difficulty is compounded by the repetition which is necessary if the bone shaping requires relocation to attain a better final position for the component. Also the interface between the component and bone will necessarily be irregular relative to the trabecular structure of the latter and this will result in the transmission of non-compressive forces through the interface during subsequent patient use of the joint, which forces can lead to component loosening. For the other type the final position for the component is determined in an effectively arbitrary manner and this cannot be reliably compatible with maintenance of normal ligament action or, therefore, joint function.